1. Field of the Invention
The present invention relates to marine seismic surveys, and in particular for a system and method for generating a route traversing survey lines of a marine seismic survey.
2. Related Art
Marine seismic surveys are used by oil companies to locate geological traps which may contain oil or gas, and to determine suitable drilling locations. In general, an oil company negotiates with governments to explore particular geographic areas. The company's geologists identify the most promising regions within these areas, and then design survey patterns that they hope will deliver the best information for the lowest cost. Survey patterns can be characterised as “2D” or “3D” survey patterns depending on the coarseness of the pattern in comparison with the features of interest. A typical “2D” survey pattern might consist of one hundred straight survey lines, ranging from 30 to 200 km in length arranged in an overlapping grid; A typical “3D” survey pattern might consist of a similar number of parallel lines more closely spaced.
The survey is carried out by a vessel or ship 101 towing one or more long subsurface streamers 102 and one or more seismic sources 103, as shown in FIG. 1. Each source 103 is an array of compressed air guns that generates an intense shock wave or seismic pulse at regular intervals of time. Each streamer 102 is a long cable containing hydrophones along its length. As the ship sails along a straight line, the source 103 produces a seismic pulse that travels through the water and is reflected back towards the surface by structural layers beneath the sea floor. The hydrdphones inside each streamer 102 detect these reflected signals, the signal reaching each hydrophone having travelled along a different path. By analysing and correlating the reflected signals detected by the hydrophones, it is possible to construct a profile of the geological structure of the sea bed along the straight line followed by the ship. A map or model of the geological structure of a particular region of the sea floor can be generated from seismic data measured along a series of straight lines forming a mesh over the region of interest. Straight lines are used to facilitate analysis of the data from the array of hydrophones.
A typical marine seismic survey pattern 200 comprises a series of straight lines 202, as shown in FIG. 2. The ship 101 towing the streamer 102 and seismic source 103 sails along each line, collecting seismic data. The process of travelling from the end of one line to the start of the next is referred to as a line change. During a line change, no seismic data is collected. It is therefore important to reduce the unproductive time spent by the ship 101 on line changes between survey lines. However, planning the order and direction in which to traverse the survey lines in order to improve productivity is not a trivial task. The ship 101 could take any one of an enormous number of possible routes along the series of lines. The problem is exacerbated by the need to ensure that the streamer 102 is straight when commencing to survey a specified survey line, and also by the minimum turn radius that the particular survey vessel can use. Moreover, if the survey area is in a tidal region or in shallow water, it may be important to allow for changing local currents and tide levels over potentially dangerous hazards. Furthermore, the ship's location at a particular time may be important, to meet a helicopter, for example. Furthermore, it may be necessary to allow time to perform maintenance or urgent repairs during line changes.